1. Field of the Invention
This invention relates to densified ceramic-metal compounds.
2. Description of the Prior Art
Ceramic materials are currently being investigated for use in high stress applications such as light weight armor tiles, and other applications that require a high wear resistance, such as water jet nozzles. Improved properties in such ceramic materials are necessary if the ceramics are to be used in such applications. Because of the high hardness of boron carbide, it is often selected for use in such applications. In order for boron carbide to be used successfully in applications requiring high wear resistance and armor applications, the hardness and toughness of boron carbide must be improved.
One approach to the improvement of ceramics is the blending of the ceramic with a metal to form a composite. Most of the work which has been conducted utilizing composites of boron carbide and metals utilize high proportions of the metal component which have resulted in reducing the hardness of the ceramic. For instance, in U.S. Pat. No. 4,704,250 there is described a process for preparing low density cermets (metal-ceramic) articles by utilizing more than 5 to about 25% by volume of titanium metal in combination with boron carbide particles. Consolidation can take place at about 1,000 degrees centigrade. Cermets are produced by the process of this patent which have densities nearly the same as densified boron carbide, nearly zero porosity, and improved toughness. In the process of this patent, the metal is admixed with the boron carbide in the form of a metal hydride.
Boron carbide based composites obtained by hot pressing and stress-relief are disclosed as the result of utilizing a complex metallic binder in Poroshk. Metall. (Kiev), (8), 66-70 (1988). Watanabe et al in Modern Developments in Powder Metallurgy, Vol. 14, pages 373 to 385 (1981) disclose metal boride additives useful to improve the density, transverse rupture strength, and Vickers hardness of titanium boride-cobalt boride sintered articles. Prior to sintering, the metal borides were ball milled to provide an intimate mixture with the ceramic powder. Kang et al in Journal of the American Ceramic Society, 72 (10) 1868-1872 (1989) disclose titanium diboride having increased sintering density by the use of a combination of 0.5% by weight iron powder and boron carbide additives.
In U.S. Pat. No. 4,718,941 and U.S. Pat. No. 4,605,440, Halverson et al describe boron carbide-aluminum composites wherein a porous ceramic precursor is infiltrated utilizing molten aluminum. Chemical treatment of the boron carbide is described in U.S. Pat. No. 4,718,941. The boron carbide is washed in a polar solvent such as an alcohol, an ester, or other polar organic solvent in order to remove boron oxide from the surface of the boron carbide. This treatment is said to render the boron carbide more amenable to infiltration by the molten metal. A typical washing process for boron carbide with an alcohol or ester or other polar organic solvent involves exposure of the boron carbide to the organic solvent for a few hours to a few days or longer. In U.S. Pat. No. 4,605,440, boron carbide-reactive metal cermets are disclosed which are produced by hot pressing to make composites having full (theoretical) density by co-dispersing the boron carbide and reactive metal powders utilizing solvents or solvents and surfactants. The use of either polar or non-polar solvents and any of anionic, cationic, nonionic, or amphoteric surfactants is disclosed.
Cermets are disclosed in U.S. Pat. No. 3,718,441 which have nearly theoretical density. Subsequent to removal under vacuum of the oxide film on the surface of the ceramic, the metal upon consolidation, using heat and pressure, flows freely into the void spaces of the ceramic. Boron carbide, aluminum oxide, and beryllium oxide in combination with a metal having a density of less than 9 grams per cubic centimeter, such as aluminum, magnesium, nickel, and chromium, are disclosed as useful in the preparation of the cermets.